Defrost control apparatus for a heat pump



Nov. 19, 1963 w. E. MAUDLIN 3,111,009 DEFROST CONTROL APPARATUS FOR AHEAT PUMP Filed May 25, 1961 FIG].

#5 71116 oar-'20:? Qatar/m.

BULB 96 E Q L; P76. 2. E Bus 98 g a; /N VE/V r01? k Wendell E. Maud/m ATTOPNE United States Patent 3,111,00? DEFROST CONTROL APPARATUS FOR AHEAT PUMP Wendell E. Maudiin, Lebanon, IntL, assignor to Stewart- WarnerCorporation, Chicago, Ill, a corporation of Virginia Filed May 25, 1961,Ser. No. 112,602 6 Claims. ((11. 62156) This invention relates to a heatpump, and more particularly, to a control arrangement therefor operableto initiate and terminate a defrost cycle automatically upon a build-upand breakdown of frost on the heat pump evaporator coil.

This invention is an improvement in the automatic defrost control shownin Patent No. 2,666,298, issued Ianuary 19, 1954, to F. M. Jones.

An object of this invention is to provide a defrost control arrangementhaving a plurality of temperature sensing means disposed at separatelocations in a heat pump operable separately to initiate and terminate adefrost cycle therefor.

Another object of this invention is to provide a refrigerant chargedpressure actuated defrost control system having a plurality ofinterconnected temperature sensing bulbs disposed at spaced locations ina heat pump operable separately to initiate and terminate a defrostcycle therefor.

Another object of this invention is to provide a defrost controlarrangement having a closed refrigerant charged pressure systemincluding a bellows and a plurality of interconnected temperaturesensing bulbs disposed at the opposite ends of an evaporator coil in aheat pump operable separately to initiate and terminate a defrost cyclefor the heat pump responsive to temperature changes therein.

These objects as well as the principle of operation of this inventionwill be more fully understood and appreciated upon a complete disclosureof a typical embodiment of this invention disclosed in the followingspecification and drawings forming a part thereof, wherein:

FIG. 1 is a generally schematic heat pump circuit having a defrostcontrol arrangement therefor forming a part of this invent-ion; and

FIG. 2 is a representative temperature vs. time graph of the varioustemperature sensing elements used in the defrost control forming a partof this invention.

The basic operation of a heat those in the 'art. A charge ofrefrigerant, changeable between its liquid and vapor states at theexpected heat pump temperatures and commercially operable pressures, issealed in a closed heat pump circuit. The v aporous refrigerant iscompressed in a compressor and directed by appropriate conduits to acondenser coil. Heat transfer between the condenser coil and its ambientfluid withdraws heat from the refrigerant to condense the refrigerant toa high pressure liquid. The liquid refrigerant is conveyed byappropriate conduits to an evaporator coil. Throttling means in thecircuit between the coils throttles the pressure or" the liquidrefrigerant to less than the vapor pressure of the refrigerant at thetemperature ambient the evaporator coil. Heat transfer between theevaporator coil and its ambient fluid boils the w pressure liquidrefrigerant to a vapor. The vaporous refrigerant is communicated fromthe evaporator coil to the compressor for recycling in the heat pump.

The heat pump circuit is commonly the same for both the heating andcooling cycles, the main difference being the location of the condenserand evaporator coils with respect to the enclosure to the heatcontrolled. On a conventional cooling cycle, the outdoor coil acts asthe condenser coil whereas the indoor coil acts as the evappump is wellknown by 3,1113% Patented Nov. 19, 1963 2 orator coil. On the otherhand, for a conventional heating cycle, the indor coil acts as thecondenser coil While re outdoor coil acts as the evaporator coil.

Frost on ice build-up occurs on the evaporator coil when the ambienttemperature thereof is approximately 40 or lower. The frost reduces heattransfer of the hot ambient air to the cool refrigerant within the coilto impede complete boiling. Consequently, the overall efii ciency of thesystem is reduced since not all of the refrigerant is boiled and thusused in the heat pump circuit.

FIG. 1 shows a closed heat pump circuit representative of a conventionalhousehold air conditioner, and includes a compressor 10 interconnectedat its outlet by conduit 12 to a 4way valve 14. Conduit 16 communicatesthe 4-way valve 14 to an accumulator 18 and thence to the inlet ofcompressor 1t Conduit 29 extends from the 4-way valve 14 to indoor coils22 or the coils located within the enclosure to be temperatureconditioned. A parallel connection extends from coils 22 through checkvalve 24, and through capil lary tubes 26 and strainer and distributor28, to T 30.

Conduit 32 connects T to T 34. Parallel lines having therein,respectively, conduit 36 and check valve 38, and conduit 4%, filterdrier 42 and expansion valve 44, in turn interconnect T 34- with T 46.Conduit 48 connects T 46 to one side of outdoor coil 50, :or the coildisposed outside of the heat controlled enclosure. Conduit 52 extendsfrom the opposite side of coil 50 and thence back to 4-way valve 14 tocomplete the heat pump circuit.

Line 54 interconnects expansion valve 44 with the outdoor coil so forcontrol of the valve by appropriate temperature or pressure means as iswell known in the art. Service valves 56 in conduits 20 and 32 permiteasy connection of the lines for installation or servicing. Sight glass58 in conduit 32 provides visual check of the refrigerant charge inliquid line 32 of the heat pump circuit.

On the cooling cycle the compressed refrigerant is directed fromcompressor 10 by the 4-way valve 14 through conduit 52 to the outdoorcoil 50. The heat transfer between the outdoor coil and the outdoor aircondenses the refrigerant to a compressed liquid. The liquid refrigerantis directed by conduit 48 through check valve 38 to T 3! The capillarytubes 26 throttle the refrigerant to a pressure lower than therefrigerant vapor pressure, the refrigerant boiling in indoor coils 22and absorbing heat from the enclosure. The vaporous refrigerant isdirected by conduit 20 to 4-way valve 14 and to the compressor forrecycling.

On the heating cycle the compressed refrigerant is directed fromcompressor 10 via 4-way valve 14 and conduit 20 to the indoor coils 22.Heat transfer with the indoor air condenses the vaporous refrigerant toa liquid. The high pressure liquid refrigerant then flows via checkvalve 24, conduit 32, through line 49 where the expansion valve 44throttles the pressure thereof. The low pressure refrigerant is directedby conduit 48 to outdoor coil 5t) where it is boiled by the heat of theoutdoor air. The vaporous refrigerant is directed by conduit 52 to 4-wayvalve 14 and thence to the compressor 10 for recycling. When the outdoorair temperature is below frost will form on the outdoor evaporator coil50.

Defrosting of the conventional heat pump as shown can be effected byreversing the 4-way valve 14 to direct the hot high pressure refrigerantgas via conduit 52 to the frosted outdoor coil 50. Coil then acts as thecondenser coil whereas indoor coil 22 acts as the evaporator coil. inessence such a defrost cycle is identical in operation to the coolingcycle of the heat pump.

This invention utilizes a defrost control similar in part to thatdisclosed in the above-mentioned Patent No.

2,666,298 to F. M. Jones. The control device 64 includes 'a lever 66fulcrumed at pivot point 68 to frame 70. Bellows 72 and 74 are supportedby the frame 70 and engage the lever 66 on opposite sides of pivot point{68 so that the bellows oppose each other. A pair of contacts 76 areactuated by rotation of the lever 66 to complete an electrical controlcircuit including control device terminals 78 and 80. A quick throwmechanism and calibration 7 means are generally included between thelever 66 and contacts 76, but are shown only schematically since theyform no part of the present invention.

, The electrical control circuit completed by contacts 76 includesappropriate conductors between the control device terminals 78 and 80,outdoor fan 82, control panel 84, and solenoid 86 on 4-way valve 14. Theelectrical control is such that during the heating cycle, one positionof contacts 76 completes control circuits to the outdoor fan '82 andsolenoid 86 to power the fan and to hold the 4-way valve 1-4 in theheating cycle. The other position of the contacts 76, corresponding to ademand for a defrost cycle, completes control circuits to the solenoid86 and fan 82 to shift 4-way valve to the defrost cycle and to stop thefan. Thus the hot refrigerant gas flow and the reduced heat transferwith the outdoor air readily melts the frost off the outdoor coil 50.

Each of the bellows 72 and 74 forms a separate pressure system chargedwith a conventional refrigerant, such as Freon 22. The pressure withineach system is determined by the temperature of the liquid surface ofthe refrigerant therein and is the vapor pressure of the refrigerantcorresponding to that temperature. Each system has temperature sensingbulb means operable to sense the temperature at particular locationswith respect to the heat pump. One sensing bulb means is disposed in theoutdoor air ambient in the evaporator outdoor coil 50, while the othersensing bulb means is strapped to the heat pump circuit to sense thetemperature of the refrigerant therein.

The difference between the sensed temperatures in the heat pump bellows74 and the ambient conditions bellows 72 changes the refrigerantpressures therein to actuate contacts 76. By proper pro-portioning ofthe bellows 72 and 74 and-their location with respect to the lever pivot'68, the necessary change in the difference of sensed temperatures canbe readily controlled. Thus reliable temperature responsive means areavailable with which to actuate an appropriate control to initiate orterminate a defrost cycle.

The improvement forming this invention includes providing a number ofseparate temperature sensing bulbs connected to the heat pump bellows ofthe control device 64. The bulbs are placed at separate locations on theheat pump circuit and in a sense act independently of each other toinitiate and terminate the defrost cycle.

Conduit 90 connects ambient bellows 72 to temperature sensing bulb 92which is disposed in the outdoor air ambient the coil 58. Conduit 94connects heat pump bellows 7 4 to temperature sensing bulbs 96 and 98.Even though the bulbs 96 and 98 are shown in a series connection withbellows 74, it will be appreciated that the bulbs could also beconnected in parallel or separately to the bellows 74, the ope-ration ofwhich would be the same as hereinafter described.

Bulb 96 is strapped or secured to the vapor end of the outdoorevaporator coil 50, as is shown on conduit 52 between the compressor 10and the coil. Bulb 98 is strapped or secured to the liquid end of theoutdoor evaporator coil as on check valve 38 between the coils 22 and50. Bulb 96 initiates the defrost cycle and has generally no function toterminate the defrost cycle, whereas bulb 98 terminates the defrostcycle and has generally no function to initiate the defrost cycle.

"FIG. 2 shows a temperature-time graph of the various temperaturesensing bulbs forming a part of the control of this invention. The graphis generally broken up into the last phases of a heating cycle, acomplete defrosting cycle, and the initial phases of a subsequentheating cycle. The temperature of the ambient outdoor air is assumedconstant and is shown as sensed by the bulb 92. Similarly, thetemperatures of the heat pump as sensed by bulbs 96 and 98 are shown bythe curves bulb 96 and bulb 98 respectively. The curves 'are merelyrepresentative of a typical heat pump but are not to be limited by theirspecific values or shapes shown since such could be readily change byconventional means known in the art.

On the heating cycle, as ice builds up on the outdoor coil 50, thereduced heat transfer of the ambient air to the coil decreases theeffective boiling of the refrigerant therein. The temperature of therefrigerant leaving coil 50 in vapor conduit 52 thus begins to decreaseand is sensed by initiating bulb 96. At the temperature corresponding tothe preset actuating pressure difference between the ambient air sensingbellows 72 and the heat pump sensing bellows 74, the control contacts 76are actuated as hereinbefore outlined. The heat pump cycle is thusswitched to a defrost cycle by the control circuit including solenoid 86of the 4-way valve '14.

Conduit 52 now carries the high pressure hot vaporous refrigerant fromcompressor 10. Temperature of the vapor line sensing bulb 96 thus risesrapidly to the temperature of the hot refrigerant gas. The refrigerantwithin bulb 96 vaporizes and migrates toward a cooler location withinthe system where it then condenses. The temperature of the refrigerantwithin the outdoor coil 50 is rapidly cooled by the ice formed on thecoil. The refrigerant in the heat pump control system thus begins tocondense in the bellows 74 and liquid line sensing bulb 98.

The ice on coil 50 rapidly melts because of the large heat quantitiesgiven off by the condensing refrigerant therein. The temperature of therefrigerant leaving coil 50 in the liquid line thus begins to rise, andterminating bulb -98 senses such rise. The vapor pressure of therefrigerant within the closed heat pump control system increases untilthe necessary predetermined pressure difference between the heat pumpsensing bellows 74 and the ambient air sensing bellows 72 is reached toactuate the control contacts 76. The control circuit including solenoid86 returns the 4-way valve 14 to resume the heat pump cycle.

Thus it is seen that the separate temperature sensing bulbs 96 and 98each control the heat pump control system pressure independently, thebulb having the lower temperature controlling the pressure within thesystem. This arrangement eliminates defects associated with existingcontrol devices, such as the above-mentioned Jones device, Patent No.2,666,298, that have only one heat pump temperature sensing bulbgenerally located intermediate the evaporator coil.

Bulb 96, located on the vapor line from coil 58, initiates the defrostcycle and is not subject to rapid false temperature changes. Thevaporous refrigerant from coil 50 is commonly superheated 10 or 12degrees above the boiling temperature at the vapor pressure in the coil.Since initiating bulb 96 will sense this superheat, control device 64can be adjusted to respond to a temperature change corresponding to thesuperheat, thus allowing a greater temperature margin before inducing adefrost cycle. This eliminates most nuisance defrost cycles caused byvariations in the heat pump cycle sufficient to vary the temperaturewithin the evaporator coil but which are not caused by frost on thecoil. Such variations can be caused by change of tolerance of theexpansion valve setting or capillary tubes, slight variations due to theleakage of the refrigerant change, variation in temperature or quantityof the air flowing over the coil, or localized temperature changes dueto localized ice formations on the coil.

Immediately following a reversal of the heat pump between the defrostcycle and a subsequent heating cycle,

there is no pressure difference across the expansion valve 44.Similarly, the pressure within the outdoor coil 50 is reducedsufficiently to vaporize the liquid refrigerant therein. Since therefrigerant in vapor conduit 52 is already a vapor, there is littletemperature change because of reduction of pressure. Thus, the nuisanceand sometimes repetitive defrost cycle associated with a single defrostcontrol located intermediate the evaporator coil is eliminated by thismulti-bulb arrangement. A rapid drop in temperature of the terminationbulb 98 does not reduce the control system pressure sufliciently toinitiate the defrost cycle.

It is again noted that the refrigerant condenses within each controlsystem at the location having the lowest temperature, and that thetemperature of the liquid refrigerant surface determines the pressurethroughout the entire system. Thus the volumes of the bellows 74,conduit 94, and bulbs 96 and 98 must be proportioned such that theliquid surface will be located within the proper bulb to issue theproper defrost control command, regardless if that is the coldestlocation in the system or not. Pressure throughout the system will bedetermined then by the liquid surface temperature in the control bulbgenerally independent of other temperature ambient the system. It hasbeen found that to accomplish the above desired relationship thetermination bulb 98 is slightly smaller than the initiating bulb 96.

While a typical embodiment and the preferred bulb locations, have beenshown, it is to be understood that various modifications can be madetherein without departing from the true spirit of the invention. Thus itis desired the invention be limited not by the arrangement shown, but bythe claims hereinafter following.

What is claimed is:

1. In a heat pump defrost control, the combination comprising means forreversing the heat pump from one heat pump cycle to the defrost cyclefor the evaporator coil of the one heat pump cycle, a pair ofrefrigerant charged systems each including a pressure bellows disposedto oppose the movement of the other, the resultant movement of which isoperable to actuate the reversing means, one of said systems having atemperature sensing bulb operable to sense the temperature of the airambient the previously mentioned evaporator coil, the other of thesystems having a pair of temperature sensing .bul bs adapted to sensethe temperature at the opposite ends of the evaporator coil, one of thebulbs being disposed in temperature sensing relationship with the vaporend of the evap orator coil operable to initiate the defrost cycle, theothers of the bulbs being disposed in temperature sensing relationshipwith the liquid end of the evaporator coil operable to terminate thedefrost cycle, said heat pump bellows and bulbs being of such size thatthe liquid surface of the refrigerant is always located in the cooler ofthe two bulbs, so that upon a suflicient change in temperature of theheat pump and ambient air, the pressure differences of the systemscorresponding thereto actuate the reversing means.

2. A defrost control for a heat pump having two coils acting as operableresponsive to temperature changes in one of the coils acting as theevaporator coil during a particular heat pump cycle to initiate andterminate a defrost cycle therefor, the combination comprising, meansfor changing from the particular heat pump cycle to the defrost cycle,biasing means responsive to the fluid temperature ambient theabove-mentioned one coil acting as the evaporator coil, a refrigerantcharged system including a pressure bellows opposing the biasing meansand a pair of temperature sensing bulbs, one of the bulbs being disposedin temperature sensing relationship with the heat pump on the vapor endof the one coil operable to initiate the defrost cycle, the others ofthe bulbs being disposed in temperature sensing relationship with theheat pump on the liquid end of the one coil operable to terminate thedefrost cycle, said bulbs and bellows being sized appropriately so thatthe liquid surface of the refrig- 6 erant therein is always located inthe cooler of the two bulbs, the vapor end bulb being slightly largerthan the liquid end bulb.

3. A defrost control device for a heat pump, comprising a frame, a pairof pressure bellows secured to the frame, a movable member operativelyconnected to the bellows and movable thereby, a temperature sensing bulbconnected to one of the bellows and defining a closed refrigerantcharged system, said bulb being adapted to sense the temperature ambientthe heat pump coil acting as the evaporator coil for a particular heatpump cycle, and first and second temperature sensing bulbs connected tothe other of the bellows and defining another closed refrigerant chargedsystem, said last-mentioned bulbs being adapted to sense the temperatureof the heat pump itself, the first bulb being disposed in temperaturesensing relationship with the heat pump on the vapor side of thepreviouslymentioned heat pump coil operable to initiate a defrost cycle,and the second bulb being disposed in temperature sensing relationshipwith the heat pump on the liquid side of the previously-mentioned heatpump coil operable to terminate the defrost cycle.

4. In a heat pump defrost control including a pair of closed refrigerantcharged systems each having a pressure bellows and temperature sensingbulb means, said bulb means being operable to sense respectively, thetemperatures of fluid ambient the heat pump evaporator coil on aparticular heat pump cycle and the heat pump itself proximate thepreviously-mentioned evaporator coil, so that upon predeterminedtemperature changes of the heat pump and ambient fluid, the pressuredifferences of the systems corresponding thereto initiate or terminateappropriately a defrost cycle in the heat pump for thepreviously-mentioned evaporator coil, the improvement comprising twotemperature sensing bulb means connected to the heat pump bellows, oneof the bulb means adapted to be disposed in temperature sensingrelationship on the vapor side of the previously mentioned evaporatorcoil operable to initiate the defrost cycle, the others of the bulbmeans adapted to be disposed in temperature sensing relationship on theliquid side of the previously mentioned evaporator coil operable toterminate the defrost cycle, said heat pump bulb means and bellows beingsized so that the liquid surface of the refrigerant is always located inthe cooler of the two bulb means, the vapor side bulb being larger thanthe liquid side bulb.

5. In a heat pump having a circuit including a compressor, a pair ofcoils, conduits between the compressor and corresponding ends of thecoils directing a refrigerant from the compressor to one of the coilsacting as the condenser coil for affecting a particular heat pump cycle,and pressure throttling means between the opposite ends of the coilcompleting the circuit and causing the other of the coils to act as theevaporator coil for the particular heat pump cycle, the improvementbeing a control for defrosting the above-mentioned other coilautomatically upon a buildup of frost thereon, comprising a pressureactuated bellows, first and second bulbs communicating with the bellowsand defining a closed refrigerant charged pressure system, said firstbulb being disposed in temperature sensing relationship with the vaporend of the other coil operable to initiate a defrost cycle, said secondbulb being disposed in temperature sensing relationship with the liquidend of the other coil operable to terminate the defrost cycle, meansactuated by the bellows operable to direct refrigerant from thecompressor to the corresponding end of the other coil to affect saiddefrost cycle, and said second bulb being disposed on the heat pumpcircuit relative to the pressure throttling means so as to be exposed onthe defrost cycle to the heat pump refrigerant confined at a pressuresubstantially the same as that to which first bulb is exposed on thedefrost cycle, and to be exposed on the particular heat pump cycle tothe refrigerant confined at pressures substantially the same as anddifferent from that to which the first bulb is exposed on the particularheat pump cycle.

6. In a heat pump having a refrigerant charged circuit including acompressor, a pair of coils, a 4-Way reversing valve between thecompressor and corresponding ends of-the coils, pressure throttlingmeans between the opposite ends of the coils, and means operable toreverse the 4-way valve for directing the heat pump refrigerant from thecompressor selectively to the vapor ends of the coils and being operableon a particular heat pump cycle to direct the refrigerant to the vaporend of one of the coils then acting as the condenser coil for that heatpump cycle, the improvement comprising a pair of refrigerant chargedsystems including opposing pressure bellows operable to actuate thereversing means, one of the systems being responsive to the temperatureambient the other of the coils while the other of the systems beingresponsive to temperature of the heat pump system itself, said secondmentioned system including first and second temperature sensing bulbsconnected to the heat pump bellows, the first bulb being disposed on theheat pump circuit between the compressor and the other coil effective toinitiate the defrost cycle to actuate the reversing means and cause areversal of the refrigerant in the heat pump circuit, the second bulbbeing disposed on the heat pump circuit proximate the opposite end ofthe other coil exposed to the heat pump refrigerant at generally thesame pressure on the defrost cycle as that of the first bulb and exposedto heat pump refrigerant at both substantially the same pressure and asubstantially higher pressure on the abovementioned particular heat pumpcycle as that of the first bulb effective to terminate the defrostcycle, and said heat pump bellows and bulbs being of such size that theliquid surface of the refrigerant is always located in the cooler of thetwo bulbs and thefirst bulb being slightly larger than the second bulb.

References Cited in the file ofthis patent UNITED STATES PATENTS2,531,136 Kurtz Nov. 21, 1950 2,666,298 Jones Jan. 19, 1954 2,940,278Thompson June 14, 1960

5. IN A HEAT PUMP HAVING A CIRCUIT INCLUDING A COMPRESSOR, A PAIR OFCOILS, CONDUITS BETWEEN THE COMPRESSOR AND CORRESPONDING ENDS OF THECOILS DIRECTING A REFRIGERANT FROM THE COMPRESSOR TO ONE OF THE COILSACTING AS THE CONDENSER COIL FOR AFFECTING A PARTICULAR HEAT PUMP CYCLE,AND PRESSURE THROTTLING MEANS BETWEEN THE OPPOSITE ENDS OF THE COILCOMPLETING THE CIRCUIT AND CAUSING THE OTHER OF THE COILS TO ACT AS THEEVAPORATOR COIL FOR THE PARTICULAR HEAT PUMP CYCLE, THE IMPROVEMENTBEING A CONTROL FOR DEFROSTING THE ABOVE-MENTIONED OTHER COILAUTOMATICALLY UPON A BUILDUP OF FROST THEREON, COMPRISING A PRESSUREACTUATED BELLOWS, FIRST AND SECOND BULBS COMMUNICATING WITH THE BELLOWSAND DEFINING A CLOSED REFRIGERANT CHARGED PRESSURE SYSTEM, SAID FIRSTBULB BEING DISPOSED IN TEMPERATURE SENSING RELATIONSHIP WITH THE VAPOREND OF THE OTHER COIL OPERABLE TO INITIATE A DEFROST CYCLE, SAID SECONDBULB BEING DISPOSED IN TEMPERATURE SENSING RELATIONSHIP WITH THE LIQUIDEND OF THE OTHER COIL OPERABLE TO TERMINATE THE DEFROST CYCLE, MEANSACTUATED BY THE BELLOWS OPERABLE TO DIRECT REFRIGERANT FROM THECOMPRESSOR TO THE CORRESPONDING END OF THE OTHER COIL TO AFFECT SAIDDEFROST CYCLE, AND SAID SECOND BULB BEING DISPOSED ON THE HEAT PUMPCIRCUIT RELATIVE TO THE PRESSURE THROTTLING MEANS SO AS TO BE EXPOSED ONTHE DEFROST CYCLE TO THE HEAT PUMP REFRIGERANT CONFINED AT A PRESSURESUBSTANTIALLY THE SAME AS THAT TO WHICH FIRST BULB IS EXPOSED ON THEDEFROST CYCLE, AND TO BE EXPOSED ON THE PARTICULAR HEAT PUMP CYCLE TOTHE REFRIGERANT CONFINED AT PRESSURES SUBSTANTIALLY THE SAME AS ANDDIFFERENT FROM THAT TO WHICH THE FIRST BULB IS EXPOSED ON THE PARTICULARHEAT PUMP CYCLE.